The present invention relates to an image forming method as well as an image forming apparatus which carries out digital exposure, and an electrostatic latent image developing toner employed by the same.
In recent years, high quality images have been increasingly demanded of image forming apparatuses such as copiers, printers, and the like which carry out digital exposure. Further, digital imaging has progressed in which electrostatic latent images are formed employing digital exposure and subsequently developed.
In common images, the ratio of an area, to which a toner is practically applied to carry out printing, is no more than 30 percent with respect to the entire image area. In digital exposure, being different from analogue exposure, it is easy to carry out exposure in which image information signals are reversed. Accordingly, from the viewpoint of print speed as well as minimization of the fatigue of a photoreceptor, it is advantageous that parts of an image are subjected to exposure and to reversal development. However, the reversal development is unstable as the development method, compared to the normal development, and as a result, it is difficult to carry out stable reversal development.
Prior to the development of a digital image, image exposure is carried out by controlling light intensity as well as the exposure time employing a semiconductor laser, LED, and the like, to form latent image dots. Accordingly, laser dots are composed of electrical potentials distributed in a normal distribution-like state. However, when the latent image dots formed in such a state are developed, it is required that the size and shape of dots in the original document are the same as those in the developing image. Specifically, when halftone, and the like, is printed, image quality is determined depending on the degree of matching the dots of the developing image to those of the original document.
When a common toner is employed, it is impossible to carry out stable development with high reproducibility for electric potential having a semi-normal distribution when such a distribution exists at development. The reasons for this are as follows. In commonly employed toner, which is prepared employing a pulverization method, fractures exist on its surface, and minute toner particles, which remain unclassified, exist, or minute toner particles, generated by stress in a development unit, remain. Accordingly, the distribution of electrostatic potential is widened, and toner having low electrostatic potential or toner components having reverse polarity, are adhered onto the edge portion of dots. Thus, it is impossible to form dots having uniform size as well as shape.
The present invention has been accomplished to provide a means to overcome the aforementioned problems.
Namely, it is an object of the present invention to provide an image forming method as well as an image forming apparatus which exhibits excellent dot reproducibility and is capable of forming high quality images, and an electrostatic latent image developing toner which is employed by the same. The invention and the embodiment thereof are described.
An image forming method wherein a latent image formed on a latent image forming body employing exposure having a exposure diameter A (in xcexcm) in the primary scanning direction is subjected to reversal development employing a developer comprising toner to form an image, and the relationship between the exposure diameter A (in xcexcm) in the primary scanning direction and the development diameter B (in xcexcm) in the primary scanning direction of the developed image is hold.
1.1xe2x89xa6B/Axe2x89xa61.5 
The toner is prepared preferably by fusing at least the resin particles in a water based medium.
It is preferred that the reversal development is carried out by the contact development, and ratio (Vs/Vp) of line velocity of a latent image forming body (Vp) to line velocity of developer carrying device (Vs) is 1.1 to 3.0.
It is preferred that the exposure diameter in the primary scanning direction is between 20 and 100 xcexcm.
It is preferred that the exposure diameter in the secondary scanning direction is between 20 and 100 xcexcm.
It is preferred that the toner is prepared by fusing at least the resin particles having weight average diameter of between 50 and 2000 nm in a water based medium.
It is preferred that the water based medium comprises at least 50 percent by weight and organic solvents of methanol, ethanol, isopropanol, butanol, acetone, methyl ethyl ketone or tetrahydrofuran.
It is preferred that after reversal development, said obtained toner image is transferred onto an image support, and subsequently fixed.
It is preferred that content ratio of said electrostatic latent image developing toner particles, having a volume average particle diameter of 3 to 9 xcexcm, a shape coefficient of 1.3 to 2.2 in the formula described below, and a shape coefficient of 1.3 to 2.0, is at least 80 percent in terms of the number of particles.
xe2x80x83Shape coefficient=(maximum diameter/2)2xc3x97xcfx80/projection area
An image forming apparatus comprising a means to uniformly charge the surface of a latent image forming body, a means to carry out digital exposure corresponding to an image to form an electrostatic latent image, a means to carry out reversal development employing a developer comprising a toner, a means to transfer an obtained toner image onto an image support, and a means to fix said toner image, wherein said toner is prepared by fusing at least the resin particles in a water based medium.
An image forming apparatus comprising a means to uniformly charge the surface of a latent image forming body, a means to carry out digital exposure corresponding to an image to form an electrostatic latent image, a means to carry out reversal development employing a developer comprising a toner, a means to transfer an obtained toner image onto an image support, and a means to fix said toner image, wherein said toner is prepared by fusing at least the resin particles in a water based medium.
An electrostatic latent image developing toner employed in an image forming method in which after uniformly charging the surface of a latent image forming body, digital exposure corresponding to an image is carried out; a formed electrostatic latent image is subjected to reversal development employing a developer comprising a toner; and an obtained toner image is transferred onto an image support and subsequently fixed, wherein the latent image developing toner is prepared by fusing at least the resin particles in a water based medium.
It is preferable that the content ratio of said electrostatic latent image developing toner particles, having a volume average particle diameter of 3 to 9 xcexcm, a shape coefficient of 1.3 to 2.2 in the formula described below, and in addition, a shape coefficient of 1.3 to 2.0, is at least 80 percent in terms of the number of particles.
Shape coefficient=[(maximum diameter/2)2xc3x97xcfx80]/projection area 
It is preferable that in said-electrostatic latent image developing toner, the content ratio of minute toner particles, having a particle diameter of no more than 3.0 xcexcm, is no more than 20 percent in terms of the number of particles.
It is preferable that the content ratio of minute toner particles, having a particle diameter of no more than 2.0 xcexcm, is no more than 10 percent in terms of the number of particles.
An image forming method wherein a latent image formed on a latent image forming body employing exposure having a exposure diameter A (in xcexcm) in the primary scanning direction is subjected to reversal development employing a developer comprising toner to form an image, and the relationship between the exposure diameter A (in xcexcm) in the primary scanning direction and the development diameter B (in xcexcm) in the primary scanning direction of the developed image, as described below, is held.
1.1xe2x89xa6B/Axe2x89xa61.5 
As described above, the present invention relates to an image forming method as well as an image forming apparatus employing an electrostatic latent image formed by an electrophotographic method and the like, and specifically to an image forming apparatus in which a latent image is formed employing a modulated beam which is obtained by being modulated with digital image data transferred from a computer and the like, and the resulting latent image is visualized.
In recent years, in the field of electrophotography, and the like, in which an electrostatic latent image is formed on a photoreceptor, and in which the resulting latent image is developed to obtain a visualized image, research and development of an image forming method utilizing a digital system has been increasingly carried out in which improvement of image quality, conversion, editing, and the like, are easily performed, and the realization of high quality images is possible. Scanning optical systems, which carry out light modulation employing digital image signals from a computer used in said image forming method or apparatus, or from an original document for copying, include an apparatus which carries out light modulation employing an acoustic optical modulator, while providing said acoustic optical modulator into the laser optical system, an method in which LED is employed as the light source, and the like. From any of these scanning systems, spot exposure is carried out onto a uniformly charged photoreceptor and halftone images are formed.
When a latent image is formed on a photoreceptor through digital exposure, a beam dot is used for scanning to give the exposure. When forming a two-dimensional image, the exposure is carried out in a way wherein scanning in a one-dimensional direction (first scanning direction) is conducted, then, a scanning position is advanced in a second scanning direction perpendicular to the above-mentioned direction to conduct the following scanning in the one-dimensional direction, so that these scanning operations are repeated. The direction for the first scanning is a primary scanning direction, and scanning conducted in a direction perpendicular to the primary scanning direction is a secondary scanning.
A length of one cycle is determined by a photoreceptor and is constant, independently of whether the photoreceptor is cylindrical or belt-shaped. However, a transfer material, such as a sheet of paper, on which an image is formed by transferring eventually takes a plurality of sizes, and its length in a longitudinal direction is different from that in a lateral direction. Therefore, a length of the transfer material is not the same as a length of one cycle of the photoreceptor. Therefore the direction in which the photoreceptor moves in advance is hard to select the primary scanning direction. Accordingly, the primary scanning is conducted in the direction perpendicular to the direction of movement of the photoreceptor in the course of exposure, while, the secondary scanning is conducted in the direction of movement of the photoreceptor. For example, the primary scanning direction, for example, of a cylindrical photoreceptor is perpendicular to the direction of rotation of the photoreceptor and the secondary scanning direction is in the direction of rotation of the photoreceptor.
A beam irradiated from aforementioned optical scanning system exhibits a circular or elliptical luminance distribution, which is similar to a normal distribution having a wide distribution range on both sides. Commonly, for example, in the case of laser beam, spots in the primary scanning direction or in the secondary scanning direction, or in both directions, on the photoreceptor, are circular or elliptical, and have an extremely small size between 20 and 100 xcexcm.
An image is formed so that the exposure diameter A (in xcexcm) in the primary scanning direction and the development diameter B (in xcexcm) preferably satisfy the relationship described below.
1.1xe2x89xa6B/Axe2x89xa61.5 
By satisfying said relationship, it is possible to produce detailed images, to obtain reproducibility of fine lines, and further to produce so-called multiple generation copies at good quality.
The exposure diameter as described herein means the maximum diameter of dots of a latent image in the primary scanning direction formed on a photoreceptor, while the development diameter, as described herein, means the maximum diameter of dots of toner formed by developing the latent image on the photoreceptor in the primary scanning direction.
Further, the exposure diameter in the primary scanning direction is generally between 20 and 100 xcexcm, and is preferably between 30 and 80 xcexcm. Various diameters may be selected based on the required definition of specific images. The exposure diameter in the secondary scanning direction is between 20 and 100 xcexcm.
In order to develop a latent image, employing the scanning digital exposure system as described in the present invention, it is important that the minute toner particle are not blended with a developer. The content of the minute particles having a diameter of no more than 3.0 xcexcm is generally no more than 20 percent by number of the entire toner particles, and preferably, the content of minute toner particle, having a diameter of no more than 2.0 xcexcm, is no more than 10 percent.
The reason for this is as follows. Even when minute toner particles are present, it is possible to carry out development with good reproducibility. However, minute toner particles exhibit high electrostatic adhesion properties and thus adhere well to the photoreceptor. As a result, the transfer properties of the toner are degraded and said minute toner particles cause non-uniform images during transfer.
The toner preferably usable for the present invention is prepared by fusing at least the resin particles in a water based medium. Since this production method includes a process in which minute particles are fused, said minute particles themselves do not remain, and further, released minute toner particles are not formed when compared to toner prepared employing a pulverization method.